Re: [PATCH][v3] KVM: x86: refine kvm_vcpu_is_preempted

Peter Zijlstra
2022-02-02T14:54:45Z

On Mon, Jan 17, 2022 at 01:37:22PM +0800, Li RongQing wrote:
> After support paravirtualized TLB shootdowns, steal_time.preempted
> includes not only KVM_VCPU_PREEMPTED, but also KVM_VCPU_FLUSH_TLB
>
> and kvm_vcpu_is_preempted should test only with KVM_VCPU_PREEMPTED

This still fails to actually explain what the problem is, why did you
write this patch?

>
> Co-developed-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Li RongQing <lirongqing@baidu.com>
> ---
> diff with v2: using andl and setnz
> diff with v1: clear 64bit rax
>
> arch/x86/kernel/kvm.c | 5 +++--
> 1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index b061d17..fe0aead 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -1025,8 +1025,9 @@ asm(
> ".type __raw_callee_save___kvm_vcpu_is_preempted, @function;"
> "__raw_callee_save___kvm_vcpu_is_preempted:"
> "movq __per_cpu_offset(,%rdi,8), %rax;"
> -"cmpb $0, " __stringify(KVM_STEAL_TIME_preempted) "+steal_time(%rax);"
> -"setne %al;"
> +"movb " __stringify(KVM_STEAL_TIME_preempted) "+steal_time(%rax), %al;"
> +"andl $" __stringify(KVM_VCPU_PREEMPTED) ", %eax;"
> +"setnz %al;"

Isn't the below the simpler way of writing that same?

diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index a438217cbfac..bc79adcf59ff 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -1025,7 +1025,7 @@ asm(
".type __raw_callee_save___kvm_vcpu_is_preempted, @function;"
"__raw_callee_save___kvm_vcpu_is_preempted:"
"movq __per_cpu_offset(,%rdi,8), %rax;"
-"cmpb $0, " __stringify(KVM_STEAL_TIME_preempted) "+steal_time(%rax);"
+"testb $" __stringify(KVM_VCPU_PREEMPTED) ", " __stringify(KVM_STEAL_TIME_preempted) "+steal_time(%rax);"
"setne %al;"
"ret;"
".size __raw_callee_save___kvm_vcpu_is_preempted, .-__raw_callee_save___kvm_vcpu_is_preempted;"